As the furthest orbiting planet in the Solar System, Neptune has the longest year, taking 164.8 Earth years to complete a single orbit of the Sun. Prior to its reclassification as a dwarf planet in 2006, Pluto was the planet with the longest year, equal to about 248 Earth years.
University Today explains that the length of a year increases in relation to distance from the Sun. The closest planet to the Sun, Mercury, has the shortest year, equivalent to 88 Earth days. A year on Venus equals 224.7 Earth days, and a year on Mars equals 687 Earth days. Jupiter's year equals 11.9 Earth years, Saturn's equals 29.7 Earth years, and Uranus' equals 84.3 Earth years.